Co2 recovery unit and co2 recovery method

ABSTRACT

A CO 2  recovery unit includes a CO 2 -absorber that causes a gas containing CO 2  to contact a CO 2 -absorbing solution and that causes the CO 2  in the gas to be absorbed into the CO 2 -absorbing solution; a CO 2 -regenerator that heats the CO 2 -absorbing solution, releases the CO 2  from the CO 2 -absorbing solution, and regenerates the CO 2 -absorbing solution; and a CO 2  recovery amount controller that: calculates a computed target value of a CO 2  recovery amount and a computed target value of a CO 2  recovery rate based on a set value of the CO 2  recovery rate, actual measured values of CO 2  concentration, gas flow rate, and temperature of the gas, and calculates a maximum value of the CO 2  recovery amount in the CO 2 -absorber and a maximum value of the CO 2  recovery amount in the CO 2 -regenerator.

TECHNICAL FIELD

The present invention relates to a CO₂ recovery unit and a CO₂ recovery method, for example, a CO₂ recovery unit and a CO₂ recovery method that recover CO₂ in a gas to be treated, using a CO₂-absorbing solution.

BACKGROUND

In the related art, CO₂ recovery units that recover CO₂ exhausted from boilers or the like of thermoelectric power plants are suggested (for example, refer to PTL 1). In the CO₂ recovery units, flue gas is introduced into a CO₂-absorber, a CO₂-absorbing solution is brought into contact with CO₂ included in the flue gas so that CO₂ is made to be absorbed thereinto. The CO₂-absorbing solution that has absorbed CO? is sent to a CO₂-regenerator and is decarboxylated with heating by a regeneration heater that regenerates the CO₂-absorbing solution, and thereby, a high-concentration CO₂ gas is recovered. The CO₂-absorbing solution after the decarboxylation is supplied to the CO₂-absorber by a liquid feed pump, and the CO₂-absorbing solution is circulated and used between the CO₂-absorber and the CO₂-regenerator.

CITATION LIST Patent Literature

[PTL 1] Japanese Patent No. 5237204

SUMMARY

Meanwhile, in a CO₂ recovery unit described in PTL 1, operation is performed in a state where a constant CO₂ recovery rate is maintained by adjusting the CO₂-absorbing solution to be supplied to the CO₂-absorber and the flow rate of saturated steam to be supplied to a regeneration heater of the CO₂-regenerator, based on the gas flow rate of the flue gas, the introduction temperature of the flue gas, or the like. However, in a case where such control is performed, a control in which the CO₂ recovery unit operates in a state where the constant CO₂ recovery rate is maintained even if the CO₂ concentration in the flue gas and the gas flow rate of the flue gas have increased is performed. Thus, for example, the load of the CO₂ recovery unit may increase if there are restrictions or the like to the amount of steam used in the CO₂ recovery unit.

One or more embodiments of the invention provide a CO₂ recovery unit and a CO₂ recovery method that enable stable operation to continue even if operation conditions have changed.

A CO₂ recovery unit in accordance with one or more embodiments includes a CO₂-absorber that brings a gas (i.e., gas to be treated) and a CO₂-absorbing solution into contact with each other to cause CO₂ included in the gas to be absorbed into the CO₂-absorbing solution; a CO₂-regenerator that heats the CO₂-absorbing solution which has absorbed CO₂, releases CO₂ from the CO₂-absorbing solution, and regenerates the CO₂-absorbing solution; and a CO₂ recovery amount controller (“controller” used interchangeably with “control unit”) that calculates a computed target value of a CO₂ recovery amount and a computed target value of a CO₂ recovery rate based on a set value of the CO₂ recovery rate, actual measured values of CO? concentration, gas flow rate, and temperature of the gas, and maximum values of the CO₂ recovery amounts in the CO₂ absorber and the CO₂-regenerator and that controls the amount of the CO₂-absorbing solution supplied to the CO₂-absorber, the amount of the CO₂-absorbing solution supplied to the CO₂-regenerator and the amount of saturated steam supplied to a regeneration heater of the CO₂-regenerator, based on the set value of the CO₂ recovery rate or the computed target value of the CO? recovery rate.

According to this CO₂ recovery unit, the circulation amount of the CO₂-absorbing solution and the amount of the saturated steam supplied to the regeneration heater can be appropriately controlled according to changes in the actual measured values of the CO₂ recovery rate and the CO₂ recovery amount in the gas. Accordingly, even when there is an influence on a predetermined relational expression to be used for control and the precision of a measuring instrument due to changes in operation conditions and the measuring instrument, the CO₂ recovery unit that can control the CO₂ recovery amount and/or the CO? recovery rate to a target value with high precision can be realized.

In the CO₂ recovery unit, the CO₂ recovery amount controller may calculate the computed target value of the CO₂ recovery rate based on the following Formulas (1) to (3).

Y1=X1×X2×X3×α  Formula (1)

Y2=min(X4, Y1)   Formula (2)

Y3=Y2/(X2×X3×α)   Formula (3)

(In Formulas (1) to (3), X1 represents the set value of the CO₂ recovery rate, X2 represents an actual measured value of the CO₂ concentration of the gas, X3 represents an actual measured value of the gas flow rate of the gas, X4 represents a maximum value of the CO₂ recovery amount, Y1 represents a target value of the CO₂ recovery amount, Y2 represents a computed target value of the CO₂ recovery amount, Y3 represents a computed target value of the CO₂ recovery rate, and a represents a conversion factor).

In the CO₂ recovery unit, the CO₂ recovery amount controller may calculate the computed target value of the CO₂ recovery rate based on the maximum value of the CO₂ recovery amount when (i.e., in a case where) a target value of the CO₂ recovery amount exceeds the maximum value of the CO₂ recovery amount, and may calculate the computed target value of the CO₂ recovery rate based on the calculated target value of the CO₂ recovery amount when the target value of the CO₂ recovery amount is equal to or lower than the maximum value of the CO₂ recovery amount.

In the CO₂ recovery unit, the CO₂ recovery amount controller may calculate the computed target value of the CO₂ recovery amount based on threshold values when the actual measured values of the gas flow rate, CO₂ concentration, and temperature of the gas exceed predetermined threshold values.

In the CO₂ recovery unit, the CO₂ recovery amount controller may feedback-control the operation of the overall device using the computed target value of the CO₂ recovery amount.

A CO₂ recovery method in accordance with one or more embodiments includes a process of bringing a gas (i.e., gas to be treated) and a CO₂-absorbing solution into contact with each other to cause CO₂ included in the gas to be absorbed into the CO₂-absorbing solution in a CO₂-absorber; and a process of heating the CO₂-absorbing solution which has absorbed CO₂, releasing CO₂ from the CO₂-absorbing solution, and regenerating the CO₂-absorbing solution in a CO₂-regenerator. A computed target value of a CO₂ recovery amount and a computed target value of a CO₂ recovery rate are calculated based on a set value of the CO₂ recovery rate, actual measured values of CO₂ concentration, gas flow rate, and temperature of the gas, and maximum values of the CO₂ recovery amounts in the CO₂-absorber and the CO₂-regenerator, and the amount of the CO₂-absorbing solution supplied to the CO₂-absorber, the amount of the CO₂-absorbing solution supplied to the CO₂-regenerator, and the amount of saturated steam supplied to a regeneration heater of the CO₂-regenerator are controlled based on the set value of the CO₂ recovery rate or the computed target value of the CO₂ recovery rate.

According to this CO₂ recovery method, the circulation amount of the CO₂-absorbing solution and the amount of the saturated steam supplied to the regeneration heater can be appropriately controlled according to changes in the actual measured values of the CO₂ recovery rate and the CO₂ recovery amount in the gas. Accordingly, even when there is an influence on a predetermined relational expression to be used for control and the precision of a measuring instrument due to changes in operation conditions and the measuring instrument, the CO₂ recovery method that can control the CO₂ recovery amount and/or the CO₂ recovery rate to a target value with high precision can be realized.

In the CO₂ recovery method, the computed target value of the CO₂ recovery rate may be calculated based on the following Formulas (1) to (3).

Y1=X1×X2×X3×α  Formula (1)

Y2=min(X4, Y1)   Formula (2)

Y3=Y2/(X2×X3×α)   Formula (3)

(In Formulas (1) to (3), X1 represents the set value of the CO₂ recovery rate, X2 represents an actual measured value of the CO₂ concentration of the gas, X3 represents an actual measured value of the gas flow rate of the gas, X4 represents a maximum value of the CO₂ recovery amount, Y1 represents a target value of the CO₂ recovery amount, Y2 represents a computed target value of the CO₂ recovery amount, Y3 represents a computed target value of the CO₂ recovery rate, and a represents a conversion factor).

In the CO₂ recovery method, the computed target value of the CO₂ recovery rate may be calculated based on a maximum value of the CO₂ recovery amount when a target value of the CO₂ recovery amount exceeds a maximum value of the CO₂ recovery amount, and may be calculated based on the target value of the CO₂ recovery amount when the target value of the CO₂ recovery amount is equal to or lower than the maximum value of the CO₂ recovery amount.

In the CO₂ recovery method, the computed target value of the CO₂ recovery amount may be calculated based on threshold values when the actual measured values of the gas flow rate, CO₂ concentration, and temperature of the gas exceed predetermined threshold values.

In the CO₂ recovery method, the operation of the overall device may be feedback-controlled using the computed target value of the CO₂ recovery amount.

Accordingly, a CO₂ recovery unit and a CO₂ recovery method in accordance with one or more embodiments enable stable operation to continue even if operation conditions have changed can be realized.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a CO₂ recovery unit in accordance with one or more embodiments of the invention.

FIG. 2 is a functional block diagram of a CO₂ recovery amount controller in accordance with one or more embodiments of the invention.

FIG. 3 is a block diagram illustrating operation control using the CO₂ recovery amount controller in accordance with one or more embodiments of the invention.

DESCRIPTION OF EMBODIMENTS

The present inventors have noted that, in a related-art CO₂ recovery unit that operates in a state where a constant CO₂ recovery rate is maintained, a control in which the CO₂ recovery unit operates in a state where the constant CO₂ recovery rate is maintained even if the CO₂ concentration in a gas to be treated and the gas flow rate of flue gas have increased may be performed, and the load of the CO₂ recovery unit may increase. The present inventors also have found out that CO₂ recovery amount calculated using a gas flowmeter and a gas concentration meter is controlled such that the CO₂ recovery amount becomes equal to or lower than a reference value, so that an increase in the load of the CO₂ recovery unit can be prevented even when operation conditions have fluctuated.

Hereinafter, embodiments of the invention will be described in detail with reference to the accompanying drawings. However, the invention is not limited to the following embodiments, and can be appropriately changed and carried out. Additionally, the configuration of the following CO₂ recovery unit can be appropriately combined and carried out.

FIG. 1 is a schematic view of a CO₂ recovery unit in accordance with one or more embodiments of the invention. As illustrated in FIG. 1, a CO₂ recovery unit 1 is an device that absorbs CO₂ in flue gas (an example of a gas to be treated) 11A containing CO₂ exhausted from industrial facilities, such as a boiler and a gas turbine, and recovers a high-concentration CO₂ gas. The CO₂ recovery unit 1 includes a quencher 12 that cools the flue gas 11A containing CO₂ exhausted from industrial facilities, such as a boiler and a gas turbine; a CO₂-absorber 14 that is provided in a subsequent stage of the quencher 12, brings the cooled flue gas 11A into contact with a CO₂-absorbing solution 13, and makes the CO₂-absorbing solution 13 absorb and remove CO₂ in the flue gas 1 1A; and a CO₂-regenerator 15 that is provided in the subsequent stage of the CO₂-absorber 14, releases CO₂ from the CO₂-absorbing solution 13 that has absorbed the CO₂, and regenerates the CO₂-absorbing solution 13.

In the CO₂ recovery unit 1, the CO₂-absorbing solution 13 circulates between the CO₂-absorber 14 and the CO₂-regenerator 15. The CO₂-absorbing solution 13 (lean solution) absorbs CO₂ in the CO₂-absorber 14, and is supplied to the CO₂-regenerator 15 as the CO₂-absorbing solution 13 (rich solution). Additionally, the CO₂-absorbing solution 13 (rich solution) has substantially all CO₂ removed and regenerated in the CO₂-regenerator 15, and is supplied to the CO₂-absorber 14 as the CO₂-absorbing solution 13 (lean solution).

The quencher 12 has a quenching section 121 that cools the flue gas 11A. A circulation line L1 is provided between a bottom part of the quencher 12 and a top part of the quenching section 121. A heat exchanger 122 that cools cooling water W1, and a circulation pump 123 that circulate the cooling water W1 within the circulation line L1 are provided in the circulation line L1.

In the quenching section 121, the flue gas 11A is cooled by bringing the flue gas 11A into countercurrent contact with the cooling water W1. The heat exchanger 122 cools the cooling water W1 heated by the heat exchange with the flue gas 11A. The circulation pump 123 supplies the cooling water W1, which has flowed down to the bottom part of the quencher 12, to the top part of the quenching section 121 via the heat exchanger 122.

The CO₂-absorber 14 includes a CO₂ absorption section 141 that is provided on a lower part side of the CO₂-absorber 14 and has the flue gas 11A cooled in the quencher 12 supplied thereto, a washing section 142 that is provided on an upper part side of the CO₂-absorber 14. A liquid storage section 144 that stores cleaning water W2 for cleaning a flue gas 11B from which CO₂ has been removed is provided at a bottom part of the washing section 142. A circulation line L2, through which the cleaning water W2, containing the CO₂-absorbing solution 13 recovered in the liquid storage section 144, is supplied and circulated from a top part side of the washing section 142, is provided between the liquid storage section 144 and an upper part of the washing section 142. The circulation line L2 is provided with a heat exchanger 21 that cools the cleaning water W2, and a circulation pump 22 that circulates the cleaning water W2, containing the CO₂-absorbing solution 13 recovered in the liquid storage section 144, within the circulation line L2 via the heat exchanger 21. Additionally, the circulation line L2 is provided with an extraction line L3 through which a portion of the cleaning water W2 (cleaning water W3) is extracted and supplied to the CO₂ absorption section 141. The extraction line L3 is provided with a control valve 23 that adjusts the amount of supply of cleaning water W3 supplied to the CO₂-absorbing solution 13 (lean solution).

In the CO₂ absorption section 141, the flue gas 11A containing CO₂ and the CO₂-absorbing solution 13 containing alkanolamine or the like come into countercurrent contact with each other. Accordingly, CO₂ in the flue gas 11A is absorbed by the CO₂-absorbing solution 13 through a chemical reaction shown in the following reaction formula. As a result, the flue gas 11A containing CO₂ becomes the flue gas 11B from which CO₂ has been removed by passing through the CO₂ absorption section 141.

R—NH₂+H₂O+CO₂→R—NH₃HCO₃

In the washing section 142, the flue gas 11B from which CO₂ has been removed rises via a chimney tray 145. Then, the flue gas 11B is brought into gas-liquid contact with the cleaning water W2 supplied from the top part side of the washing section 142, and becomes a flue gas 11 C from which the CO₂-absorbing solution 13 entrained in the flue gas 11B has been recovered by circulation cleaning. The flue gas 11C is exhausted to the outside from a tower top part 14 a of the CO₂-absorber 14 after mist in the gas is trapped by a mist eliminator 146.

A rich solution supply tube 50 through which the CO₂-absorbing solution 13 (rich solution), which has absorbed CO₂ in the CO₂-absorber 14, is supplied to an upper part side of the CO₂-regenerator 15, is provided between a tower bottom part 14 b of the CO₂-absorber 14 and an upper part of the CO₂-regenerator 15. The rich solution supply tube 50 is provided with a rich solvent pump 51 that supplies the CO₂-absorbing solution 13 (rich solution), which has absorbed CO₂ in the CO₂-absorber 14, toward the CO₂-regenerator 15, and a rich-lean solution heat exchanger 52 that heats the CO₂-absorbing solution 13 (rich solution) that has absorbed CO₂, using the CO₂-absorbing solution 13 (lean solution) which has been heated with saturated steam and from which CO₂ has been removed.

The CO₂-regenerator 15 includes a CO₂-absorbing solution supply part 151 that is provided at a central part of the CO₂-regenerator 15 and has the CO₂-absorbing solution 13, which has absorbed CO₂, supplied thereto, and a mirror surface part 152 of a tower bottom part 15 b of a lower part of the CO₂-absorbing solution supply part 151. The tower bottom part 15 b of the CO₂-regenerator 15 is provided with a circulation line L4 through which the CO₂-absorbing solution 13 that has flowed down to the tower bottom part 15 b circulates. The circulation line L4 is provided with a regeneration heater 31 that heats the CO₂-absorbing solution 13 with saturated steam S.

A tower top part 15 a of the CO₂-regenerator 15 is provided with a gas exhaust line L5 through which a CO₂ gas 41 accompanied by the saturated steam S is exhausted. The gas exhaust line L5 is provided with a condenser 42 that condenses moisture in the CO₂ gas 41, and a separation drum 43 that separates the CO₂ gas 41 from condensed water W5. The separation drum 43 separates the condensed water W5 from the CO₂ gas 41, and releases a CO₂ gas 44, from which the condensed water W5 has been separated, from an upper part of the separation drum 43 to the outside. A condensed water line L6 through which the condensed water W5 separated by the separation drum 43 is supplied to the upper part of the CO₂-regenerator 15 is provided between a bottom part of the separation drum 43 and the upper part of the CO₂-regenerator 15. The condensed water line L6 is provided with a condensed water circulation pump 45 that supplies the condensed water W5 separated by the separation drum 43 to the upper part of the CO₂-regenerator 15.

Additionally, the tower bottom part 15 b of the CO₂-regenerator 15 and an upper part of the CO₂ absorption section 141 of the CO₂-absorber 14 are provided with a lean solution supply tube 53 through which the CO₂-absorbing solution 13 (lean solution) in the tower bottom part 15 b of the CO₂-regenerator 15 is supplied to the upper part of the CO₂ absorption section 141. The lean solution supply tube 53 is provided with the rich-lean solution heat exchanger 52 that heats the CO₂-absorbing solution 13 (rich solution), which has absorbed CO₂, using the CO₂-absorbing solution 13 (lean solution) which has been heated with the saturated steam S and from which CO₂ has been removed, a lean solution pump 54 that supplies the CO₂-absorbing solution 13 (lean solution) in the tower bottom part 15 b of the CO₂-regenerator 15 to the upper part of the CO₂ absorption section 141, and a quenching section 55 that cools the CO₂-absorbing solution 13 (lean solution) to a predetermined temperature.

The CO₂ recovery unit 1 in accordance with one or more embodiments includes a flue gas detecting unit 101 that is provided in a flow passage for the flue gas 11A to be introduced into the quencher 12, a CO₂ concentration meter 102 that is provided in a flow passage for the flue gas 11C exhausted from CO₂-absorber 14, a CO₂ gas detecting unit 103 that is provided in a flow passage for the CO₂ gas 44 exhausted from the separation drum 43, and a concentration meter 104 that measures the concentration of the CO₂-absorbing solution (lean solution) 13 to be supplied to the CO₂-absorber 14.

A flue gas detecting unit 101 a measures CO₂ concentration in the flue gas 11A to be introduced into the quencher 12, and transmits the measured CO₂ concentration to a CO₂ recovery amount controller 111. Additionally, a flue gas detecting unit 101 b measures the gas flow rate and the gas temperature of the flue gas 11 A, and transmits the gas flow rate and the gas temperature to the CO₂ recovery amount controller 111. The CO₂ concentration meter 102 detects CO₂ concentration in the flue gas 11C exhausted from the CO₂-absorber 14, and transmits the detected CO₂ concentration to the CO₂ recovery amount controller 111.

The CO₂ gas detecting unit 103 detects the gas flow rate and the concentration of the CO₂ gas 44 exhausted from the separation drum 43, and transmits the gas flow rate and the concentration to the CO₂ recovery amount controller 111. The concentration meter 104 measures the concentration of the CO₂-absorbing solution (lean solution) 13 to be supplied to the CO₂-absorber 14, and transmits the measured concentration of the CO₂-absorbing solution (lean solution) 13 to the CO₂ recovery amount controller 111.

The CO₂ recovery amount controller 111 calculates a computed target value of the CO₂ recovery rate based on a set value of the CO₂ recovery rate, actual measured values of the CO₂ concentration, gas flow rate, and temperature of the flue gas 11A, and a maximum value of the CO₂ recovery amount of the overall device. Additionally, the CO₂ recovery amount controller 111 controls the amount of the CO₂-absorbing solution 13 supplied to the CO₂-absorber 14 and the amount of the CO₂-absorbing solution 13 supplied to the CO₂-regenerator 15 based on the calculated computed target value of the CO₂ recovery rate, and controls the amount of the saturated steam S supplied to the regeneration heater 31 of the CO₂-regenerator 15.

FIG. 2 is a functional block diagram of the CO₂ recovery amount controller 111 in accordance with one or more embodiments of the invention. The CO₂ recovery amount controller 111 in accordance with one or more embodiments includes a calculating unit 112, and a flue gas controller 113, an absorbing solution controller 114, and a steam controller 115. The calculating unit 112 calculates the maximum value of the CO₂ recovery amount that is determined according to device design conditions and utility conditions of steam or the like and is capable being recovered by the CO₂ recovery unit 1. Additionally, the calculating unit 112 calculates a target value of the CO₂ recovery amount based on the set value of the CO₂ recovery rate that is set in advance, the CO₂ concentration in the flue gas 11 A measured by the flue gas detecting unit 101 a, and an actual measured value of the gas flow rate of the flue gas 11A measured by the flue gas detecting unit 101 b. The target value of the CO₂ recovery amount is a CO₂ recovery amount that is determined based on the set value of the CO₂ recovery rate that is set in advance. Moreover, the calculating unit 112 calculates the computed target value of the CO₂ recovery amount based on the target value of the CO₂ recovery amount and the calculated maximum value of the CO₂ recovery amount. The computed target value of the CO₂ recovery amount is, for example, any smaller value of the maximum value of the CO₂ recovery amount or and the target value of the CO₂ recovery amount. Additionally, the calculating unit 112 calculates the computed target value of the CO₂ recovery rate based on the calculated computed target value of the CO₂ recovery amount, the CO₂ concentration in the flue gas 11 A, and the actual measured value of the gas flow rate of the flue gas 11A. The computed target value of the CO₂ recovery rate is a value of the CO₂ recovery rate for realizing a CO₂ recovery amount according to actual operation conditions or the like of the CO₂ recovery unit 1. Moreover, the calculating unit 112 transmits the calculated computed target values of the CO₂ recovery rate and the CO₂ recovery amount to the flue gas controller 113, the absorbing solution controller 114, and the steam controller 115.

The calculating unit 112 calculates the computed target value that is the target value of the CO₂ recovery rate of the overall device based on various kinds of input data and various actual measured values. In one or more embodiments, the calculating unit 112 calculates the computed target value of the CO₂ recovery rate, for example, based on the following Formulas (1) to (3). In addition, □ is any value that is determined depending on design conditions or the like of the CO₂ recovery unit.

Y1=X1×X2×X3×α  Formula (1)

Y2=min(X4, Y1)   Formula (2)

Y3=Y2/(X2×X3×α)   Formula (3)

(In Formulas (1) to (3), X1 represents the set value of the CO₂ recovery rate, X2 represents the actual measured value of the CO₂ concentration of the flue gas, X3 represents the actual measured value of the gas flow rate of the flue gas, X4 represents the maximum value of the CO₂ recovery amount, Y1 represents the target value of the CO₂ recovery amount, Y2 represents the computed target value of the CO₂ recovery amount, Y3 represents the computed target value of the CO₂ recovery rate, and □ represents a conversion factor).

The flue gas controller 113 controls the flow rate of the flue gas 11A to be introduced into the quencher 12 via a control valve V1 based on the set value and the computed target value of the CO₂ recovery rate calculated by the calculating unit 112.

The absorbing solution controller 114 controls the liquid volume of the CO₂-absorbing solution (lean solution) 13 to be supplied to the CO₂-absorber 14 via a control valve V2 based on the set value and the computed target value of the CO₂ recovery rate calculated by the calculating unit 112, and controls the liquid volume of the CO₂-absorbing solution (rich solution) 13 to be supplied to the CO₂-regenerator 15 via a control valve V3. The steam controller 115 controls the flow rate of the saturated steam S to be supplied to the regeneration heater 31 via a control valve V4 based on the set value and the computed target value of the CO₂ recovery rate calculated by the calculating unit 112.

In one or more embodiments, the calculating unit 112 calculates the computed target value of the CO₂ recovery rate based on the maximum value of the CO₂ recovery amount when the target value of the CO₂ recovery amount exceeds the maximum value of the CO₂ recovery amount, and calculates the computed target value of the CO₂ recovery rate based on the target value of the CO₂ recovery amount when the target value of the CO₂ recovery amount is equal to or lower than the maximum value of the CO₂ recovery amount. Accordingly, even when operation conditions, such as the CO₂ concentration, gas flow rate, and temperature of the flue gas 11A, fluctuate, and CO₂ introduced into CO₂ recovery unit 1 has increased, the CO₂ recovery unit 1 can be stably operated because the overall device can be controlled based on the set value of the CO₂ recovery rate at which the CO₂ recovery amount becomes equal to or lower than a maximum value capable of being processed by the overall device. Additionally, when CO₂ introduced into the CO₂ recovery unit 1 becomes equal to or lower than the maximum value of the CO₂ recovery amount, the overall device can be controlled based on the set value of the CO₂ recovery rate that is set in advance. Thus, an operational state according to a design can be brought about.

Next, the overall operation of the CO₂ recovery unit 1 in accordance with one or more embodiments will be described. The flue gas 11A containing CO₂ exhausted from industrial facilities, such as a boiler and a gas turbine, is introduced into the quencher 12, and is brought into countercurrent contact with and cooled by the cooling water W1 after the CO₂ concentration, gas flow rate, and temperature in the flue gas 11A are measured by the flue gas detecting unit 101. The cooled flue gas 11 A is introduced into the CO₂-absorber 14 via a flue 16. The flue gas 11A introduced into the CO₂-absorber 14 is brought into countercurrent contact with the CO₂-absorbing solution 13 containing alkanolamine or the like in the CO₂ absorption section 141, and becomes the flue gas 11B from which CO₂ in the flue gas 11A has been absorbed by the CO₂-absorbing solution 13 and CO₂ has been removed.

The flue gas 11B from which CO₂ has been removed rises via the chimney tray 145, is brought into gas-liquid contact with the cleaning water W2 supplied from the top part side of the washing section 142, and becomes the flue gas 11C from which the CO₂-absorbing solution 13 entrained in the flue gas 11 B has been recovered by circulation cleaning. The CO₂ concentration in the flue gas 11 C is measured by the CO₂ concentration meter 102 and the flue gas 11 C is exhausted from the tower top part 14 a of the CO₂-absorber 14 to the outside, after the mist in the gas is caught by the mist eliminator 146.

The CO₂-absorbing solution 13 (rich solution) that has absorbed CO₂ is sent to the rich-lean solution heat exchanger 52 by a rich solvent pump 51 via a rich solution supply tube 50 in the CO₂-absorber 14. In the rich-lean solution heat exchanger 52, the CO₂-absorbing solution 13 (rich solution) sent from the CO₂-absorber 14 is heat-exchanged with the CO₂-absorbing solution 13 (lean solution) sent from the CO₂-regenerator 15. The CO₂-absorbing solution 13 (rich solution) after this heat exchange is supplied to the upper part of the CO₂-regenerator 15. The CO₂-absorbing solution 13 supplied to the CO₂-regenerator 15 has CO₂ removed therefrom and becomes a semi-lean solution, while flowing down to the tower bottom part 15 b via the CO₂-absorbing solution supply part 151. This semi-lean solution is circulated through the circulation line L4, is heated by the saturated steam S in the regeneration heater 31, and becomes the CO₂-absorbing solution 13 (lean solution). The saturated steam S after being heated becomes the saturated steam condensed water W4. The CO₂ gas 41 removed from the CO₂-absorbing solution 13 is released to the outside as the CO₂ gas 44 from which the condensed water W5 has been separated through the upper part of the separation drum 43 after the moisture thereof is condensed by the condenser 42. As for the CO₂ gas 44, CO₂ concentration in the CO₂ gas 44 is measured by the CO₂ gas detecting unit 103.

The CO₂-absorbing solution 13 (lean solution) of the tower bottom part 15 b of the CO₂-regenerator 15 is supplied to the upper part of the CO₂ absorption section 141 of the CO₂-absorber 14 by the lean solution pump 54 after being heat-exchanged with the CO₂-absorbing solution 13 (rich solution) by the rich-lean solution heat exchanger 52 via the lean solution supply tube 53.

FIG. 3 is a flow chart illustrating operation control using the CO₂ recovery amount controller 111 in accordance with one or more embodiments. As illustrated in FIG. 3, the CO₂ recovery amount controller 111 calculates the target value of the CO₂ recovery amount based on various kinds of input data, such as the CO₂ set value that is set in advance, the CO₂ concentration in the flue gas 11 A, and the flow rate and temperature of the flue gas 11A (Step ST1), and determines whether or not the calculated target value of the CO₂ recovery amount is equal to or lower than the maximum value of the CO₂ recovery amount (Step ST2). Then, when the target value of the CO₂ recovery amount is equal to or lower than the maximum value of the CO₂ recovery amount, the CO₂ recovery amount controller 111 controls the flow rate of the CO₂-absorbing solution 13 and the flow rate of the saturated steam S to be supplied to the regeneration heater 31 via the absorbing solution controller 114 and the steam controller 115, based on the set value of the CO₂ recovery rate (Step ST3A). Additionally, when the target value of the CO₂ recovery amount exceeds the maximum value of the CO₂ recovery amount, the CO₂ recovery amount controller 111 controls the flow rate of the CO₂-absorbing solution 13 and the flow rate of the saturated steam S to be supplied to the regeneration heater 31 via the absorbing solution controller 114 and the steam controller 115, based on the computed target value of the CO₂ recovery rate (Step ST3B).

As described above, according to one or more embodiments, the circulation amount of the CO₂-absorbing solution 13 and the amount of the saturated steam S supplied to the regeneration heater 31 can be appropriately controlled according to changes in the target values of the CO₂ recovery rate and the CO₂ recovery amount in the flue gas 11A. Accordingly, for example, even when there are changes in operation conditions, such as a case where CO₂ concentration in flue gas has increased, or even when there is an influence on a predetermined relational expression to be used for operation control and the precision of a measuring instrument due to change of the measuring instrument, or the like, the CO₂ recovery unit 1 that can control the CO₂ recovery amount and/or the CO₂ recovery rate to a target value with high precision can be realized.

In addition, in the above-described embodiments, the CO₂ recovery amount controller 111 may continuously monitor the gas flow rate, CO₂ concentration, and temperature of the flue gas 11A, thereby calculating the computed target value of the CO₂ recovery rate. However, the present invention is not limited to this configuration. For example, the CO₂ recovery amount controller 111 may control the CO₂ recovery amount to a value equal to or lower than the computed target value of the CO₂ recovery amount when the actual measured values of the gas flow rate, CO₂ concentration, and temperature of the flue gas 11A exceed predetermined threshold values. By performing such control, for example, even when a malfunction has occurred in measuring instruments, such as a CO₂ concentration meter and a gas flowmeter, it is possible to operate the CO₂ recovery unit 1 appropriately.

Additionally, in the above-described embodiments, the CO₂ recovery amount controller 111 may feedback-control the operation of the overall device using the computed target value of the CO₂ recovery amount. By performing such control, the operation of the CO₂ recovery unit 1 can be controlled based on the CO₂ recovery amount in which the response of numerical fluctuations is quick relative to the CO₂ recovery rate. Thus, it is possible to control the operation of the CO₂ recovery unit 1 more stably.

In addition, in the above-described embodiments, the flue gas 11A containing CO₂ exhausted from industrial facilities, such as a boiler and a gas turbine, is treated by the CO₂-absorbing solution 13. However, the gas to be treated is not limited to the flue gas 11A; in other embodiments of the invention, various other gases containing CO₂ may be treated by the CO₂-absorbing solution 13.

REFERENCE SIGNS LIST

-   1: CO₂ RECOVERY UNIT -   11A, 11B, 11C: FLUE GAS -   12: QUENCHER -   121: QUENCHING SECTION -   122: HEAT EXCHANGER -   123: CIRCULATION PUMP -   13: CO₂-absorbing solution -   14: CO₂-ABSORBER -   14 a : TOWER TOP PART -   14 b : TOWER BOTTOM PART -   141: CO₂ ABSORPTION SECTION -   142: WASHING SECTION -   144: LIQUID STORAGE SECTION -   145: CHIMNEY TRAY -   146: MIST ELIMINATOR -   15: CO₂-regenerator -   15 a : TOWER TOP PART -   15 b : TOWER BOTTOM. PART -   151: CO₂-absorbing solution SUPPLY PART -   152: MIRROR SURFACE PART -   16: FLUE -   21: HEAT EXCHANGER -   22: CIRCULATION PUMP -   23: CONTROL VALVE -   31: REGENERATION HEATER -   41, 44: CO₂ GAS -   42: CONDENSER -   43: SEPARATION DRUM -   45: CONDENSED WATER CIRCULATION PUMP -   50: RICH SOLUTION SUPPLY TUBE -   51: RICH SOLVENT PUMP -   52: RICH-LEAN SOLUTION HEAT EXCHANGER -   53: LEAN SOLUTION SUPPLY TUBE -   54: LEAN SOLUTION PUMP -   55: QUENCHING SECTION -   101 a, 101 b : FLUE GAS DETECTING UNIT -   102: CO₂ CONCENTRATION METER -   103: CO₂ GAS DETECTING UNIT -   104: CONCENTRATION METER -   111: CO₂ RECOVERY AMOUNT CONTROLLER -   112: CALCULATING UNIT -   113: FLUE GAS CONTROLLER -   114: ABSORBING SOLUTION CONTROLLER -   115: STEAM CONTROLLER -   L1, L2, L4: CIRCULATION LINE -   L3: EXTRACTION LINE -   L5: GAS EXHAUST LINE -   L6: CONDENSED WATER LINE -   S: SATURATED STEAM -   W1: COOLING WATER -   W2, W3: CLEANING WATER -   W4: SATURATED STEAM CONDENSED WATER -   W5: CONDENSED WATER

While the disclosure includes a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of the present disclosure. Accordingly, the scope should be limited only by the attached claims. Further, one of ordinary skill in the art would appreciate that the various “units” disclosed herein may be implemented by software or hardware (e.g., circuit). 

What is claimed:
 1. A CO₂ recovery unit comprising: a CO₂-absorber in which a gas containing CO₂ contacts a CO₂-absorbing solution that absorbs the CO₂ in the gas; a CO₂-regenerator that heats the CO₂-absorbing solution that has absorbed the CO₂ from the gas, releases the CO₂ from the CO₂-absorbing solution, and regenerates the CO₂-absorbing solution; and a CO₂ recovery amount controller that: calculates a computed target value of a CO₂ recovery amount and a computed target value of a CO₂ recovery rate based on a set value of the CO₂ recovery rate, actual measured values of CO₂ concentration, gas flow rate, and temperature of the gas, calculates a maximum value of the CO₂ recovery amount in the CO₂-absorber and a maximum value of the CO₂ recovery amount in the CO₂-regenerator, and controls an amount of the CO₂-absorbing solution supplied to the CO₂-absorber, an amount of the CO₂-absorbing solution supplied to the CO₂-regenerator, and an amount of saturated steam supplied to a regeneration heater of the CO₂-regenerator based on the set value of the CO₂ recovery rate or the computed target value of the CO₂ recovery rate, wherein the CO₂ recovery amount controller controls the CO₂ recovery amount to a value less than or equal to a computed target value of the CO₂ recovery amount when any one of the actual measured values of the CO₂ concentration, gas flow rate, and temperature of the gas exceeds a predetermined threshold value.
 2. The CO₂ recovery unit according to claim 1, wherein the CO₂ recovery amount controller calculates the computed target value of the CO₂ recovery rate based on the following formulas (1) to (3): Y1=X1×X2×X3×α  Formula (1) Y2=min(X4, Y1)   Formula (2) Y3=Y2/(X2×X3×α)   Formula (3) wherein, X1 represents the set value of the CO₂ recovery rate, X2 represents an actual measured value of the CO₂ concentration of the gas, X3 represents an actual measured value of the gas flow rate of the gas, X4 represents a maximum value of the CO₂ recovery amount, Y1 represents a target value of the CO₂ recovery amount, Y2 represents a computed target value of the CO₂ recovery amount, Y3 represents a computed target value of the CO₂ recovery rate, and a represents a conversion factor.
 3. The CO₂ recovery unit according to claim 1, wherein the CO₂ recovery amount controller: calculates the computed target value of the CO₂ recovery rate based on the maximum value of the CO₂ recovery amount when a target value of the CO₂ recovery amount exceeds a maximum value of the CO₂ recovery amount, and calculates the computed target value of the CO₂ recovery rate based on the calculated target value of the CO₂ recovery amount when the target value of the CO₂ recovery amount is less than or equal to the maximum value of the CO₂ recovery amount.
 4. The CO₂ recovery unit according to claim 1, wherein the CO₂ recovery amount controller feedback-controls an operation of the CO₂ recovery unit using the computed target value of the CO₂ recovery amount.
 5. A CO₂ recovery method comprising: heating a CO₂-absorbing solution that has absorbed CO₂ from a gas in a CO₂-absorber, releasing the CO₂ from the CO₂-absorbing solution, and regenerating the CO₂-absorbing solution in a CO₂-regenerator; calculating a computed target value of a CO₂ recovery amount and a computed target value of a CO₂ recovery rate based on a set value of the CO₂ recovery rate, actual measured values of CO₂ concentration, gas flow rate, and temperature of the gas; calculating a maximum value of the CO₂ recovery amount in the CO₂-absorber and a maximum value of the CO₂ recovery amount in the CO₂-regenerator; and controlling an amount of the CO₂-absorbing solution supplied to the CO₂-absorber, an amount of the CO₂-absorbing solution supplied to the CO₂-regenerator, and an amount of saturated steam supplied to a regeneration heater of the CO₂-regenerator based on the set value of the CO₂ recovery rate or the computed target value of the CO₂ recovery rate; and wherein the CO₂ recovery amount is controlled to a value less than or equal to a computed target value of the CO₂ recovery amount when any one of the actual measured values of the CO₂ concentration, gas flow rate, and temperature of the gas exceeds a predetermined threshold value.
 6. The CO₂ recovery method according to claim 5, wherein the computed target value of the CO₂ recovery rate is calculated based on the following formulas (1) to (3): Y1=X1×X2×X3×α  Formula (1) Y2=min(X4, Y1)   Formula (2) Y3=Y2/(X2×X3×α)   Formula (3) wherein, X1 represents the set value of the CO₂ recovery rate, X2 represents an actual measured value of the CO₂ concentration of the gas, X3 represents an actual measured value of the gas flow rate of the gas to be treated, X4 represents a maximum value of the CO₂ recovery amount, Y1 represents a target value of the CO₂ recovery amount, Y2 represents a computed target value of the CO₂ recovery amount, Y3 represents a computed target value of the CO₂ recovery rate, and a represents a conversion factor.
 7. The CO₂ recovery method according to claim 5, wherein the computed target value of the CO₂ recovery rate is calculated based on a maximum value of the CO₂ recovery amount when a target value of the CO₂ recovery amount exceeds a maximum value of the CO₂ recovery amount; and the computed target value of the CO₂ recovery rate is calculated based on the target value of the CO₂ recovery amount when the target value of the CO₂ recovery amount is less than or equal to the maximum value of the CO₂ recovery amount.
 8. The CO₂ recovery method according to claim 5, wherein the method is performed by a CO₂ recovery unit and the method further comprises feedback-controlling an operation of the CO₂ recovery unit using the computed target value of the CO₂ recovery amount. 